The entire weight of the torso is transferred through the legs and into the feet during bipedal standing and walking. Therefore, the femur in bipeds is one of the most critical links between the pelvis, vertebral column, and lower legs. The femur is also the distal attachment point for the gluteal muscles that provide the propulsive force for locomotion.

The rounded femoral head articulates with the pelvis at the acetabulum (hip joint). The femoral shaft is generally straight, ending in two bulbous condyles. These condyles are larger and more elliptical in bipeds when compared to the relatively smaller and rounder condyles seen in quadrupeds. The distal end of the femur articulates with the tibia (lower leg) and patella (knee cap) at the knee joint.

The amount of force exerted on the hip joint and the femoral head increases as the acetabulum moves further away from the body’s center of gravity. The size of the femoral head reflects the amount of force absorbed at the hip joint. A femoral head with a larger diameter is able to absorb more stress. Another adaptation to counteract the increased stress on the hip joint is a longer femoral neck, which increases the mechanical advantage of the lesser gluteal muscles by lengthening their lever arm. 

Australopithecus has a relatively smaller femoral head and longer femoral neck compared to later Homo who have a relatively and absolutely larger femoral head9,17